Since the discovery of the pancreatic secretagogue secretin by W. M. BAYLISS and E. H. STARLING at the inception of the twentieth century, intense interest has focused on numerous, continually expanding classes of small peptides which appear to serve as regulatory molecules in the gastrointestinal tract, brain, and other organ systems. Initially, many of these substances were, like secretin, discovered in functional assays as "factors" or "activities" extractable in minute quantities from tissues or tissue fluids. By the middle of the century, advances in biochemical and immunological methods...